The field of the invention is that of electrical contact materials, and the invention relates more particularly to metal-metal oxide contact materials adapted to display substantial electrical conductivity while also displaying resistance to contact erosion and contact welding over a long service life.
Electrical contact materials intended for high quality, long life performance in make and break devices and the like commonly comprise metal oxide particles dispersed in a matrix of a metal, such as silver, having high electrical conductivity. The presence of the metal oxide particles substantially increases the ability of the electrical contacts to resist welding together during opening and closing of electrical circuits. The presence of the metal oxide particles also reduces erosion of the contact surfaces during circuit opening and closing and extends the service life of the contacts. Some common metal-metal oxide materials of this type include silver cadmium oxide contact materials as shown in U.S. Pat. No. 2,932,595 and silver tin-indium oxide contact materials as shown in U.S. Pat. No. 3,933,485. It is common practice to bond a thin layer of a malleable and easily weldable or brazeable material of high electrical conductivity, such as fine silver, to one surface of the metal-metal oxide material for use in attaching the contact materials to contact arms and the like.
Metal-metal oxide contact materials are made by a variety of conventional processes. Typically, however, such known manufacturing procedures or contact materials are less than fully satisfactory for various reasons.
In one known procedure, for example, a compacted mixture of silver and metal oxide powders is sintered to form the desired contact materials. However, it is difficult to provide such contact materials with full density, and contact materials with less than full density do not display satisfactory uniformity of conductivity and service life.
In another known procedure, silver alloys with selected concentrations of cadmium, tin-indium or other oxide-forming constituents are bonded to a fine silver backing layer to form a composite. In that procedure, the cadmium, tin-indium or other oxide-forming constituents of the alloys, are selected and incorporated in particular concentrations in the alloys such that the alloys are internally oxidizable under conveniently selected internal oxidizing condition. The composite is then subjected to those selected oxidizing conditions to internally oxidize the cadmium or tin-indium constituents of the alloy layer. During that treatment, oxygen penetrates the silver materials from both sides thereof and a dispersal of cadmium oxide particles or the like is formed in situ in the silver alloy layer. Typically, however, there is some migration of the cadmium or other oxide-forming constituent of the alloy layer toward the two opposite external surfaces of the composite which are exposed to the oxidizing conditions with the result that the oxide-forming constituent is depleted in a central zone in the alloy before it is internally oxidized. As a result the dispersal of metal oxides does not extend through the material but leaves a centrally located internal oxide depletion zone. If the contact material is expected to undergo substantial contact erosion, there may be concern that the service life of the contact material may be shortened.
A number of known processes have been proposed or used to deal with the problem of such internal oxide depletion zones. In one procedure believed to be in common use for dealing with internal oxide depletion zones, two sheets of a silver cadmium alloy or similar material are hermetically sealed together along the edges of the two sheets. The resulting package is then exposed to internal oxidizing conditions so that the silver cadmium layers of the sheets are each internally oxidized from the outer surfaces inward leaving an oxide-free layer in each sheet adjacent the innermost surfaces of the sheets in the package. The sheets are then cut along their edges and separated to provide two contact materials, each being substantially free of an internal oxide depletion zone with an oxide-free surface region provided as a means of attachment. However, significant manufacturing cost is involved in securing the sheets together and then separating them, and there tends to be a waste of processed material along the secured edges of the sheets during separating of the two sheets after internal oxidation thereof.
In another process, layers of silver have been bonded to both outer surfaces of a silver cadmium metal alloy sheet or the like and the resulting composite has been exposed to selected oxidizing conditions for internally oxidizing the silver cadmium alloy layer. This procedure results in a centrally located oxide-free zone of the composite which is free of metal oxide particles, and the composite has been cut in half along its central axis so that the oxide-free zone is removed as the composite is cut in half producing separate sheets of internally oxidized contact material each having a fine silver backing layer to aid in attachment. Again, the cost of cutting the composite lengthwise of its core has been considered to add significantly to manufacturing expense.
In another process, a layer of nickel is bonded to one side of a silver cadmium alloy layer to prevent oxygen penetration of the silver cadmium alloy layer from that side of the composite, thereby to prevent occurrence of a centrally located internal oxide depletion zone. The oxidation process is terminated to leave an oxide-free zone adjacent the nickel layer. However, subsequent removal of the nickel layer to expose the unoxidized silver alloy portion as a backing layer for use in brazing the contact material to a support had been considered to add significantly to manufacturing expense for the noted process to be commercially practical.